The development of the ion implantation industry continues to increase demand for systems having higher ion beam current capacity, which has led to performance challenges such as instability and slow responsiveness. In the manufacture of semiconductor devices in the past, semiconductor wafers were modified by diffusing or implanting positive or negative ions (dopants), formed from precursors such as boron, phosphorus, arsenic, antimony into the body of the wafer to produce regions having varying conductivity. Various ion implanters are known, using several types of ion sources. In most ion sources, an ion beam of a preselected chemical species is generated by means of the current applied to a filament within an ion source chamber coupled to a power supply and fitted with an ion precursor gas feed. The ions are extracted in the form of an ion beam through an aperture in the ion source chamber by means of a potential between the source chamber, which is positive, and an extraction means. The beam is directed to an acceleration system, a magnetic analysis stage that separates the desired ions from unwanted ions on the basis of mass, and a post acceleration stage that ensures delivery of the required ions at the required beam current level to the target or substrate wafer to be implanted. The size and intensity of the generated ion beam can be tailored by system design and operating conditions; for example, the current applied to the filament can be varied to regulate the intensity of the ion beam emitted from the ion source chamber.
The most common type of ion sources used for ion implanters are a Freeman source and a Bernas source. In the Freeman source, a rod-like filament, or cathode, made of tungsten or tungsten alloy is passed into an ion chamber (sometimes known as an “arc chamber”) whose walls are the anode. The ion chamber includes a gas feed for delivering the desired gas into the chamber for use in generating the desired ions, a filament supply for heating the filament to about 2000 degrees Kelvin up to about 2800 degrees Kelvin to emit thermal electrons, and an exit aperture. A magnet is provided that applies a magnetic field parallel to the filament to increase the electron path length and to suspend the plasma (ions and electrons) within the chamber. Numerous other features and aspects of the Freeman type ion chamber are shown and described in U.S. Pat. No. 4,754,200, the teachings of which are incorporated herein by reference.
The Bernas type ion chamber is substantially identical to the Freeman-type chamber and differs primarily in that the Bernas chamber uses a filament in the form of a loop at one end of the ion chamber in contrast to the rod-like filament that extends through the Freeman ion chamber. Other aspects of the Bernas ion chamber are shown and described in U.S. Pat. No. 5,262,652, the teachings of which are incorporated herein by reference.
In both the Freeman and the Bernas ion chambers, when power is supplied to the filament, the filament temperature increases until electrons are emitted that bombard and breakup the precursor gas molecules such that a plasma is formed containing the electrons and various ions. The ions are emitted from the ion source chamber through the exit aperture and are selectively passed to the target as the ion beam.
For stable ion source operation in Freeman and Bernas sources, the arc voltage and the arc current are required to be relatively constant. This was accomplished through control of the arc current by a feedback loop that controlled filament power, as explained in detail in the '200 and '652 patents. If the arc current dropped, an arc current measuring circuit together with arc current error circuit would alter the filament power control circuit to bring the actual arc current back to the level demanded. Because of the thermal inertia of a high-resistance filament, the filament power control loop for maintaining constant arc current responds relatively slowly to changes in programming.
An attempt to overcome these problems used an ion implanter having an ion chamber that operated to maintain the ion beam current by varying the arc voltage on the filament cathode while supplying direct current electrical power across the filament cathode (as described in the '200 patent). In particular, the '200 patent teaches a scheme of control that varies the arc voltage to provide a faster response while using a slower servo on the filament to maintain the beam current at the desired value.
This ion chamber is not ideal for large ranges of control since the ionization efficiency depends on the arc voltage, and the beam current extracted from the source will not remain constant if the arc voltage is varied over a wide range even if the arc current is kept constant.
The mirror electrodes in the prior art are connected to the filament cathode or are left floating. In either case, the mirror electrode charges up to the filament potential and the electrons from the filament do not have the energy to reach the electrode.
Thus, what is needed is an ion source that can rapidly control the number of electrons available for ionization.